Not applicable.
Not applicable.
The present invention relates to calenders in general, and to supercalenders in particular.
A calender, particularly a supercalender, can increase the value of the paper manufactured on a papermaking machine without increasing the cost of fiber, and with only a small increase in energy cost. By improving the surface finish or other attributes of the paper web, the value of the paper is increased without otherwise modifying the papermaking machinery or process. Because of the large fixed costs and high production rates typically involved in paper manufacture, increasing the value of the paper produced can be a particularly advantageous way to increase revenue produced by a papermaking machine.
A supercalender is comprised of a stack of rolls, sometimes as many as ten, eleven, or more, which form a plurality of nips through which the paper web is directed. Pressure and often heat are applied to the web as it passes through the nips of the supercalender. A supercalender can impart an improved, or more valuable surface finish, can correct curl, and can improve paper caliper variations.
Improving the supercalender has involved controlling the nip force between adjacent rolls by supporting each roll independently of the other rolls in the stack of rolls; the use of crown control rolls, and the use of higher roll temperatures. The use of higher roll temperatures requires an ability to rapidly open a calender stack so that the high-temperature rolls do not overheat opposed compliant rolls when a paper break occurs.
Where a plurality of intermediate rolls are mounted between a fixedly mounted, variable-crown upper roll and a movable variable-crown lower roll, one known technique for controlling inter roll nip loading is to mount the intermediate roll bearings on pivot arms. The pivot arms can be supported by support cylinders as disclosed in U.S. Pat. No. 4,901,637 to Hagel et al.; U.S. Pat. No. 5,438,920 to Koivukunnas et al.; U.S. Pat. No. 5,806,415 to Lipponen et al.; and U.S. application Ser. No. 09/303,587 (PCT/FI98/00392), filed May 7, 1998, claiming priority from U.S. provisional application 60/045,871 to Maenpaa et al., which are each incorporated herein by reference. The support cylinders allow control of the nip loading between each of the supercalender rolls.
A supercalender may employ rolls of varying diameters and of different types. One type of roll has a polymer roll cover. The resilient roll cover provides a wider nip due to compression of the roll at the nip between rolls. Polymer covered rolls have a relatively long life and require only relatively small reductions in diameter due to refinishing the roll surface during the life of the roll. Smooth metal rolls provide a hard smooth surface against which the paper is compressed. Although metal rolls may be refinished, relatively little material is removed over time. Metal rolls may be heated, typically by hot water, steam or induction heating. Another type of known roll is a filled roll which is comprised of a large number of disks of a material like cotton, flax, or paper. Each disk has a central hole and thousands of individual disks or sheets are stacked up on a metal core and compressed axially at very high pressures. The resulting roll is finished by turning the surface of the roll formed by the compressed disks of fabric or paper. The surface of a filled roll has a relatively short service life requiring frequent machining so that a filled roll decreases substantially in diameter over the life of the roll.
Many existing calenders are of the closed frame, or A-frame type, which means the roll bearings at the ends of the individual calender rolls making up the supercalender are held between pairs of vertical frames, which are joined at the top. In these existing calenders, the rolls have bearings which slide on rails between the vertical frames. Nip loading between rolls making up the calender can be controlled only by loading the uppermost roll, which means each successively lower nip has an increased nip loading as the weight of each successive roll adds to the total nip load.
A conventional closed calender cannot rapidly open the nips. Rapid nip opening protects polymer and fiber rolls from damage caused by wads of paper passing through the calender nips. Typically photoeye and web tension sensors detect a paper break and instigate rapid nip opening so that wads of paper formed during a break can pass between calender rolls without damaging them. Existing solutions to rebuilding calenders do allow support of individual rolls by hydraulic pistons which extend between a support frame and the roll bearings. Existing systems, however, do not provide sufficient vertical movement of the roll bearings to accommodate a variety of roll diameters, particularly the ability to accommodate the diameter change of filled rolls over time.
A calender or calender rebuild design is needed which can accommodate a wide variety of calender rolls, and facilitate the use of filled rolls by accommodating the substantial change in roll diameter overtime.
The calender of this invention may be based on an existing calender of the closed A-frame type. One half of each A-frame in the machine direction is removed and a weldment is bolted to the track of each remaining frame along which the bearing housings of the calender rolls formally rode. Each weldment rests on the calender foundation and consists of two parallel plates which extend in the machine direction 72 inches away from the remaining frames. The lower portion of each weldment has a vertical rail along which the bearing housings of a bottom roll rides. The bottom roll mounted to the bottom bearing is supported by a bottom cylinder which controls the bottom roll""s vertical movement and the opening and closing of the calender roll stack.
A top calender roll is fixedly mounted between the weldments. A plurality of intermediate calender rolls are mounted by pivot arms to the weldments, so that each intermediate calender roll is supported on each end by two pivoting arms. Each arm has two plates which extend between the roll end bearing, and extend along either side of the weldment to bearing pins located adjacent to the upstream side of the weldment where the weldment is bolted to the track of the existing frames.
The bearing housings of each roll connect the two plates of each arm to form a single integrated pivot arm. The bearing housings incorporate a stop so that each bearing housing on each pair of pivot arms, when pivoting downwardly comes to rest on resilient pads mounted to weldment stops which extend like teeth from the sides of the weldments. The weldment is substantially open ended, opposite the calender rolls.
Positioned within the sides of the weldments are pairs of load supporting cylinders which extend between cylinder brackets which span the sides of the weldments and piston mounting brackets which extend from the calender roll bearing housings. The piston mounting brackets are narrower than the weldment and fit within the sides of the weldment and between the weldment stops on which rubber pads are mounted, thus accommodating the stroke of the load supporting cylinders without interference of the supporting weldment.
The greater length of the pivot arms combined with the greater stroke of the load support cylinders allows the supercalender to accommodate filled rolls which change diameter substantially over their life, as the surface of the rolls is repeatedly turned down to refurbish the roll surface.
It is an object of the present invention to provide a supercalender which can accommodate calender rolls of varying diameter.
It is another object of the present invention to provide a supercalender in which greater vertical motion of individual calender rolls is provided for.
It is a further object of the present invention to provide a supercalender which can control the nip load on intermediate calender rolls.
It is a still further object of the present invention to provide a supercalender in which intermediate rolls are mounted on pivot arms which minimize lateral displacement of the rolls when they are pivoted on the arms.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.